Hydraulic pressing device and method for operating the same.
The invention relates in the first place to a method for operating a hydraulic pressing device having a stationary part and a moving part, the moving part being displaced in relation to the stationary part until a predetermined pressure is reached.
Hand-operated or motor-driven hydraulic tools are often employed for certain joining procedures, such as for example the pressing-on of cable eyes onto electrical conductors, or for riveted connections. These tools are provided with an excess pressure valve which limits the oil pressure, and thus the compressive force of the moving part against the workpiece to be pressed, to a maximum value. In order to ensure a well-made joint, e.g. of a cable eye to an electrical conductor, it is known for the excess pressure valve to act only when a prescribed minimum compressive force is reached. This makes sure that the full required compressive force was effective. After release of the excess pressure valve, the pressing device, or rather the moving part thereof, is returned manually to the initial position, i.e. the open position.
Having regard to the above-described state of the art, the invention is addressed to the technical problem of providing a method for operating a hydraulic pressing device of the type under discussion, such as for example a cable eye (connector) pressing device, or a riveting tool pressing device, in which method the handling aspect is especially improved.
This problem is solved in the first place and to a substantial extent by the subject matter of claim 1, it being provided that the moving part returns automatically and completely back into its initial position, released by the predetermined pressure being reached. Accordingly, on achievement of a maximum pressure, there occurs an automatic opening of the pressing device and complete return of the moving part into the initial position. The user is spared having to intervene manually in order to open and return the moving part. The user is simultaneously given an optical signal also, by the return movement of the moving part, that the joint has been properly made with the prescribed maximum pressure.
The invention relates moreover to a method for operating a hydraulic pressing device, such as for example a pipe clamping tool, that has a stationary part and a moving part and an automatically actuating return valve, the moving part being biased into its initial position by means of a return spring. In this type of hydraulic pressing device, especially a pipe clamping tool, it is known for the return stroke of the hydraulic piston to take place automatically once the switch-off pressure has been reached. Only for emergencies is an additional, manually operated, return valve provided. In normal operation, the pressing or the joint formation can only be ended after the maximum pressure has been exceeded and the tool opens by the automatically returning hydraulic piston which carries the moving part with it. Known constructional solutions of the desired manner of operation consist of using an excess pressure valve which, after release, is arrested by a mechanical stop mechanism and thus permits complete return travel of the spring-loaded piston. On renewed actuation of the pressing device, for example when the motor is switched on, the arresting is mechanically disconnected and the excess pressure valve falls back into the closed position. In order to provide a hydraulic pressing device of the general type here under discussion, such as for example a pipe clamping device, which is characterised in particular by an advantageous arrangement from the handling aspect, it is provided that the return valve is held open by the force of the return spring, and returns automatically to its initial closed position after removal of the restoring force. As a result of this method according to the invention, no mechanical components are required for arrest of the piston or the moving part in the initial position, which furthermore obviates structural solutions for disconnecting the arresting when the pressing device is in use. In the method according to the invention, the restoring force, which is present anyway, of the return spring, is advantageously used for returning the moving part, so as to keep the return valve open over the entire return stroke of the moving part. No further arrest means are needed. After completion of the return movement, there is no further hydraulic pressure, owing to a limiting abutment of the spring-biased hydraulic piston, which results automatically in a return displacement of the return valve into the initial, closed position. The limiting abutment of the hydraulic piston also gives rise, shortly before an end position, to the smallest hydraulic pressure effective to keep the return valve open.
The invention relates further to a hydraulic pressing device with a stationary part and a moving part, the moving part being displaced relative to the stationary part by a hydraulic piston and being movable back to an initial position by means of a return spring, the return displacement being releasable in dependence on a predetermined pressure by actuation of a return valve. In order to provide a hydraulic pressing device of the kind under discussion with improved functional reliability and handling properties, it is proposed that the automatically acting return valve be retained in the open position, throughout the entire return stroke of the hydraulic piston, by the pressure of the returning oil. Mechanical arrest of the return valve during the return stroke of the hydraulic piston or of the moving part can be dispensed with by means of this arrangement, which offers special advantages in operation. Known structural solutions consist, for example, in the use of an excess pressure valve which, after actuation, is arrested by a mechanical stop mechanism and thus makes possible a full reverse stroke of the spring-loaded piston. On a fresh actuation of the pressing tool, for example by switching the motor on, the arresting is mechanically disconnected and the excess pressure valve goes back into the closed condition. But in accordance with the invention, the pressure of the returning oil, which is present anyway, after actuation of the return valve, is used for maintaining the return valve in the open position. The return valve acts automatically when a predetermined oil pressure is exceeded. The pressure of the returning oil, which decreases during the return of the hydraulic piston, is sufficiently high over the entire return stroke to keep the return valve in the open condition. It is found especially advantageous in this connection for the return valve to be formed as a valve piston, a partial piston surface area, effective in the closed condition, being calculated having regard to the maximum pressure. To this end, the return valve consists for preference of a valve piston having for example a needle point which closes off a bore connecting with the pressure space. The smaller partial piston surface effective by reason of the bore diameter is engaged by the oil in the course of pressing by the hydraulic pressing device. If the oil pressure exceeds a value predefined by the bore diameter, the valve piston of the return valve is raised from its sealing seat by way of the partial piston surface, whereupon a substantially greater piston area comes into effect. The return valve in this position operates with a substantially lower limiting pressure than in the closed condition. The limiting pressure in this position is no longer defined by the smaller partial piston surface area, but rather by the total surface area of the valve piston, formed, as it is, as a longitudinally sliding piston. As an example, a ratio of 400:1 can exist between the total piston area and a smaller, partial piston surface area which co-operates with the sealing seat. In consequence, the limiting pressure in the open position of the valve piston is 400 times smaller than the actuation pressure in the seated position, i.e. in the initial closed condition. As a result of this arrangement, a return valve is provided which has high hysteresis, so that the valve piston remains in the open position throughout the entire return stroke of the hydraulic piston because of the oil pressure acting on the valve piston, despite the fact that the oil pressure continuously diminishes in the course of the return displacement. The valve piston only falls back into the initial closed position when the oil pressure falls below a prescribed minimum. This very low oil pressure equates to the fully returned position of the hydraulic piston. In an exemplary embodiment, actuation, i.e. the opening of the return valve, can occur at 600 bar, and automatic return travel thereof into the initial position at 1.5 bar. In a hydraulic pressing device of the kind under discussion, in which the return valve is biased into the closed position by means of a compression spring, it is provided furthermore that the cylinder in which the valve piston is accommodated, has a discharge port to an oil reservoir, and that the discharge port is opened in the course of a displacement of the valve piston into the open position. By this arrangement according to the invention, when the actuation pressure, defined by the force of the compression spring and the smaller, partial piston surface area, has been exceeded, the oil does not flow away directly into the oil reservoir. Rather the oil reaches the oil reservoir only through the discharge port, and the discharge port is not opened until the valve piston has been displaced to a overlapping extent. The valve piston, to this end, can be fitted without much play, so that relatively little oil can escape by flowing past it. In order to give the return valve controllable damping, it is provided in an advantageous development of the invention that a relief bore is provided to the rear of the valve piston, the bore penetrating the cylinder wall. This bore serves for pressure relief of the space behind the piston, and the damping of the valve can be controlled by way of the size and situation of the relief bore. The desired automatic return of the hydraulic piston is made possible, in simply-acting cylinders having a return spring, by making this return spring of such dimensions that by pressing on the hydraulic piston, it creates an oil pressure which lies above the limiting pressure of the return valve in its longitudinal sliding condition. By this means, the valve is held open and the hydraulic piston returned. The arrangement of the return spring is preferably such that the hydraulic piston returns all the way to an abutment stop. In this end position, the return flow ceases and the valve piston descends into its initial position, the seated position, after which the hydraulic pressing device is ready for the next working cycle. As an example, a target maximum pressure of 600 bar in the pressure space may be desired. If this is exceeded, the return valve is actuated and the limiting pressure sinks to about 1.5 bar. The rating of the return spring is, for instance, such that the pressure in the pressure space is always 2.5 bar during the return stroke of the hydraulic piston. The pressure difference of at least 1 bar is mainly absorbed as a throttle loss in the flow through the small bore of the sealing seat, which bore co-operates, in the closed position, with the smaller, partial piston surface; this pressure difference determines the throughflow of oil and thereby the return velocity of the hydraulic piston. One advantage of the return valve described is that along with the excess pressure valve, which has to be provided anyway, no additional parts, such as for example mechanical latching elements, are necessary. Moreover the valve goes automatically, without necessity for manual unlatching, back to its initial state. In a further embodiment, it is provided that the valve piston can be displaced into an open position by hand. Such hand operation is desired for instance for interrupting the pressing procedure. To this end, the valve piston is moved into an open position, whereby the discharge port to the oil reservoir is opened. This results in a fall in the oil pressure and thereby a return displacement of the hydraulic piston. A pulling part is furthermore of advantage, connected to the valve piston and passing through the cylinder. This pulling part, in a preferred embodiment, is movable by hand, by means of an actuating rocker. This actuating rocker constitutes for the user an advantageous lever arm, by means of which the valve piston can be lifted up from the valve seating against the force of the compression spring which acts on the seating. This arrangement in accordance with the invention ensures at all times a manual return stroke of the hydraulic piston, necessary in emergencies. In a further embodiment, it is provided that the valve piston is pot-shaped on its rear side relative to the surface of the valve piston which is exposed to pressure. It is furthermore proposed that the pulling part comprises a drive head, which is in engagement with a drive nose on the valve piston. On tripping the actuating rocker, the valve piston is accordingly displaced away from the valve seating by means of the pulling part which has the drive head. The compression spring, which co-operates in defining the actuation pressure, can here bear directly on the pot-shaped valve piston. An embodiment is preferred, however, in which the compression spring acts on the valve piston by way of the pulling part. A further advantage, especially in assembly or repair operations, arises from the drive nose being a spring washer disposed in the pot wall of the valve piston. An alternative provision has the drive nose integrally formed as a radial collar on the pot wall of the valve piston, preferably the inner wall thereof, the collar acting at the same time to centre the pulling part within the valve piston. Furthermore, the bore which co-operates with the smaller piston surface of the valve piston can be provided in a seating disc screwed-in into the cylinder. Resulting from this arrangement, it is possible in very simple manner to change the seating disc and the valve piston, the piston being associated with the pulling part by way of the spring washer. A further possibility is for the other end of the compression spring which acts on the valve piston to be supported against a screw, by which the desired preloading of the compression spring can be adjusted. Adjustment of the limiting pressure is thereby enabled. It is provided in addition that the drive head is formed as a circumferential flange on the cylindrical pulling part. It proves advantageous, moreover, especially with a view to a high functional reliability, for the pulling part to act on the valve piston by way of a spigot portion of reduced diameter located at the centre of the drive head. By these means, in very simple manner, there is achieved a moment-free transfer of force from the pulling part to the valve piston. Alternatively or in combination with the above-described embodiments, it can also be provided that the drive head, in the open condition of the return valve, defines an active piston surface. From this, it follows that the oil flowing in, after the valve is opened, acts directly on the pulling part, the drive head of the pulling part thus constituting the piston surface or at least a part thereof. Here it is preferred that the drive head, in the open condition of the return valve, constitutes a partial piston surface integrated into the active valve piston surface of the valve piston. As a result of this arrangement, the drive head, or the active piston surface thereof, is an integral component of the total piston surface, it being preferred furthermore that the valve piston surface, which is now formed to be annular, is flush with the piston surface of the drive head, at least in the operating condition. In this connection, an arrangement is preferred in which the valve piston is a hollow cylinder having a circular cross-section, the resulting annular front surface constituting the valve piston surface. It is provided in a further variation of this embodiment that the drive head provides, in the closed condition, a partial piston surface, the area of which is calculated with reference to the desired maximum pressure. To this end, the drive head is preferably equipped with a closure member, e.g. a needle point, which closes off a bore communicating with the pressure space. If the oil pressure exceeds a value predefined by the bore diameter, the drive head of the pulling part, and with it the valve piston, are lifted from the sealing seat by the partial piston surface formed by the needle point, whereupon the substantially greater piston surface provided by the valve piston and the drive head comes into action. It is furthermore proposed that the diameter of the discharge port be smaller than the height of a closed circumference of the valve piston. For preference in this connection, the diameter of the discharge port is smaller than the height of a closed circumference of the valve piston which faces the valve piston surface, so that the discharge port is initially opened to the extent of a crack and that only after the valve piston is first lifted. It is additionally proposed that, to the rear of the valve piston surface, an annular groove, open to the outer pot sleeve, be provided. This groove stands preferably at least partly in communication with the discharge port in the closed condition of the valve. The arrangement is chosen in such a way that, with raising of the valve piston, there occurs opening of the discharge port for outflow of the oil, while at the same time the open annular groove is cut off from the discharge port. This is achieved by making the distance between the valve piston surface and the annular groove greater than the diameter of the discharge port. A further provision is that an axially aligned flow passage extends from the valve piston surface, to connect the valve piston surface with the annular groove. This flow passage serves in the first place to permit outflow of the unavoidable oil residues in the stationary, closed operating condition, without the prior occurrence thereby of a pressure rise in the remaining gap. In the second place, the flow passage is kept so small that when the valve is opened, displacement of the piston takes place, since in that way the outflowing oil likewise leads to a closure of the flow passage, because of the quantity. In this, there proves to be a significant advantage in that, by reason of the connection of the valve piston surfaces with the rearwardly disposed annular groove which takes up residual quantities of oil, the oil pressure in the course of the valve closing procedure drops off rapidly, which leads in consequence to a more rapid closing of the valve. In this connection, it is further provided that the diameter of the flow passage is smaller than the diameter of an oil inlet bore of the valve. It is also proposed that the annular groove be formed in the outer wall of the pot, for a substantially horizontal disposition of the cylinder bore. An alternative proposal is that the annular groove is formed in the cylinder bore, and that the valve piston has an associated, radial bore. The latter is in communication with the valve piston surface by way of the axially aligned flow passage. The annular groove formed in the bore of the cylinder is for preference provided at the level of the discharge port. Furthermore, it is conceivable for both the cylinder bore and the exterior wall of the pot to be each equipped with a respective annular groove for the uptake of oil residues.
As an alternative or in combination, it is also conceivable to permit only partial return travel of the hydraulic piston. In this case, the rating of the return spring is such that its force in a particular position within the working stroke of the hydraulic cylinder is no longer sufficient to keep the return valve open.